sm-notify: Support creating a PF_INET6 socket in smn_create_socket()

sm-notify: Support creating a PF_INET6 socket in smn_create_socket()

Socket creation is unfortunately complicated by the need to handle the
case where sm-notify is built with IPv6 support, but the local system
has disabled it entirely at run-time (ie, socket(3) returns
EAFNOSUPPORT when we try to create an AF_INET6 socket).

The run-time address family setting is made available in the global
variable nsm_family.  This setting can control the family of the
socket's bind address and what kind of addresses we want returned by
smn_lookup().  Support for that is added in subsequent patches.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>